SDWebImage is a powerful library that enables UIImageView to load remote images from the internet. It offers features such as cache management, asynchronous downloading, control over repeated downloads for the same URL, and performance optimization. This makes it an efficient solution for handling image loading in iOS applications.
**SDWebImage Cache Flow Chart**
**Independent Asynchronous Image Download**
If you want to perform an independent asynchronous image download, you can use the `downloadImageWithURL:options:progress:completed:` method. This allows you to track the download progress and handle the completion of the download. The progress block can be used to update a progress bar, while the completion block can be used to display the downloaded image or handle any errors.
**Independent Asynchronous Image Cache**
The `SDImageCache` class is a singleton that manages both memory and disk caching. You can access it using `[SDImageCache sharedImageCache]`. Images are stored using the URL as the key. When retrieving an image, the library first checks the memory cache, and if not found, it looks into the disk cache using the MD5 hash of the URL.
**How to Find and Cache an Image**
To retrieve an image from the cache:
```objective-c
UIImage *cacheImage = [imageCache imageFromKey:myCacheKey];
```
To store an image in the cache:
```objective-c
[imageCache storeImage:myImage forKey:myCacheKey];
```
By default, images are stored in both memory and disk. If you only want to store in memory, you can set the third parameter to `NO` in the `storeImage:forKey:toDisk:` method.
**Main Components Used**
1. **UIImageView (WebCache):** This category provides methods to load images asynchronously and handles callbacks after the image is loaded.
2. **SDWebImageManager:** Acts as a central manager that coordinates between the cache and the downloader. It handles image requests, manages ongoing operations, and forwards results to the appropriate delegates.
3. **SDImageCache:** Manages both memory and disk caches. It stores images with the URL as the key in memory, and uses the MD5 hash of the URL on disk. It also handles cache cleanup when necessary.
**SDWebImage Image Loading Process**
1. The `setImageWithURL:placeholderImage:options:` method is called, which displays a placeholder image immediately.
2. The `SDWebImageManager` processes the request based on the provided URL.
3. It first checks the `SDImageCache` to see if the image is already cached.
4. If the image is found in memory, the delegate is notified, and the image is displayed.
5. If not found in memory, the system checks the disk cache using the MD5 hash of the URL.
6. If the image is found on disk, it is loaded into memory and displayed.
7. If the image is not found in either cache, a new download is initiated.
8. The `SDWebImageDownloader` is used to fetch the image from the network.
9. The download is handled via `NSURLConnection`, which tracks the progress and handles completion or failure.
10. Once the image is downloaded, it is decoded using `ImageIO`.
11. Decoding occurs in a background operation queue to prevent blocking the main thread.
12. After decoding, the image is returned to the main thread, and the UI is updated.
13. The image is then saved to both memory and disk caches for future use.
14. The cache also includes automatic cleanup mechanisms for low memory situations and when the app enters the background.
This comprehensive approach ensures that image loading is both fast and efficient, improving the overall user experience in iOS apps.
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